Particle delivery

ABSTRACT

A needleless syringe, in which particles of a therapeutic agent are entrained in a high pressure gas flow, has a nozzle surrounded by a shroud silencer through which gas reflected from the target surface may be vented to atmosphere while retaining any particles reflected in the gas.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of International PatentApplication Number PCT/GB97/00734, filed Mar. 17 1997, designating theUnited States, from which priority is claimed pursuant to 35 U.S.C.§365(c) and which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates generally to a needleless syringe for usein delivery of particles of a therapeutic agent to a target surface.More particularly, the invention is drawn to a needleless syringe systemthat is configured for delivery of particles of a therapeutic agent froma nozzle surrounded by a shroud that comprises a porous silencingmaterial that allows discharged gas to vent to atmosphere but retainsany particles reflected from the target surface within the confines ofthe shroud.

BACKGROUND OF THE INVENTION

In commonly owned U.S. Pat. No. 5,630,796, a noninvasive delivery systemis described that entails the use of a needleless syringe device. Thesyringe is used for transdermal delivery of powdered therapeuticcompounds and compositions to skin, muscle, blood or lymph. The syringecan also be used in conjunction with surgery to deliver therapeutics toorgan surfaces, solid tumors and/or to surgical cavities (e.g., tumorbeds or cavities after tumor resection).

The needleless syringe is constructed as an elongate tubular nozzle.Particles of a powdered therapeutic agent are located adjacent to theupstream end of the nozzle. The mechanics at the upstream end of thenozzle may involve a reservoir of gas such as helium, at a high pressure(e.g., between about 30 and 80 bar), and a high pressure gas flow iscreated by releasing the gas suddenly from the reservoir (e.g., byopening a valve), or by releasing the gas from the reservoir so thatpressure builds up behind a membrane at the entrance to the nozzle untilthe membrane eventually ruptures to release the high speed gas flow. Theparticles may be contained initially within the high pressure gasreservoir or within a cassette from which the particles are releasedinto the gas flow. The passageway through the nozzle itself ispreferably convergent/divergent or convergent/cylindrical, with theupstream convergent section shorter than the downstream divergent orcylindrical section.

Since the gas pressure release from the above-described needlelesssyringe device is generally at or approaching supersonic speeds and canfurther involve the rupture of one or several membranes, the actualoperation of the device can be quite noisy. In order to address thisissue, the device was provided with a tubular shroud having acylindrical silencer portion surrounding a downstream section of thenozzle. A spacer portion of the shroud extends axially beyond the exitplane of the nozzle to abut the target surface and positively space theexit plane of the nozzle from the target surface. An annular chamber wascreated between the nozzle and cylindrical silencer portion, wherein thechamber includes a series of baffles to create a tortuous path forescaping gas to pass through on its way through an upstream end of thesilencer portion where several ports were provided for venting the gasto the atmosphere. Such a shroud is illustrated in FIGS. 1 to 3 of U.S.Pat. No. 5,630,796. Although practical for its intended purpose, such ashroud is still noisy owing to the comparatively large vent ports. Inaddition, it has been found that the tubular shroud configuration allowsfor particles reflected from the target surface to pass out of the portseven after multiple reflections from the series of baffles. Accordingly,there remains a need to provide a silencer mechanism that reduces thenoise associated with operation of the needleless syringe and whichcontains any particles which may reflect from the target surface.

SUMMARY OF THE INVENTION

The present invention is particularly concerned with a novel andimproved silencer mechanism provided at the downstream end of the nozzleof a needleless syringe device such as that described in U.S. Pat. No.5,630,796. In this regard, it is desirable to dissipate andsubstantially silence both the sound pressure wave (noise) and gas flowwhich are discharged from the nozzle and which rebound from the targetsurface. This sound and pressure attenuation is effected by way of ajudicious selection of device component configurations and silencingmaterials which together provide for the quiet and efficient operationof a needleless syringe. More particularly, the downstream constructionof the device can provide for reactive attenuation of the sound andpressure (reflective attenuation structurally provided by a shroud orhousing which surrounds the nozzle), dissipative attenuation of thesound and pressure (absorbent attenuation provided by a porous silencingmaterial component of the shroud or housing, or attenuation provided bybaffles), or a combination thereof. However, any system which includes asilencing element will have an inherent backpressure which is related tothe amount of attenuation provided by the silencing element.Accordingly, the amount of reactive and/or dissipative attenuationprovided by the present invention is carefully balanced in order toavoid creating an excessive increase in backpressure which couldsubstantially affect the delivery performance of the device. Thus, thepresent attenuation mechanism readily allows for the externaldissipation of the discharged gas pressure to avoid syringe recoil andlifting of the syringe away from the target. Furthermore, theattenuation mechanism is constructed so as to contain any and all of thesmall proportion of particles or particle fragments which may bereflected from the target surface, and thus prevents their uncontrolledrelease from the syringe.

It is thus a primary object of the invention to provide a silencingmechanism for use with a needleless syringe for delivering particles toa target surface. The syringe comprises an elongate nozzle having anupstream end and a downstream end. A means for releasing a high pressureflow of gas into the upstream end of the nozzle is provided, whereinparticles entrained within the released gas flow pass through the nozzleand exit from the downstream end thereof for delivery to the targetsurface. In this regard, the nozzle is, or is arranged to be, connectedat its upstream end to a source of high pressure gas. A source ofparticles of a powdered therapeutic agent is also provided at oradjacent to the upstream end of the nozzle. The particles can bedisposed within a sealed cassette behind rupturable membranes, within acapsule, or contained within a high pressure gas reservoir. Theseparticle retaining sources are described in U.S. Pat. No. 5,630,796, andin commonly owned International Patent Application Nos. PCT/GB95/01948,and PCT/GB95/02498, the disclosures of which are incorporated herein byreference. During operation of the needleless syringe, the gas flow isreleased by or through the particle source to entrain the particles.

The silencing mechanism is provided by a tubular shroud which surroundsthe downstream end of the nozzle. The shroud has a downstream portionwhich extends beyond the downstream end of the nozzle and engages thetarget surface to establish a positive spacing between the nozzle andthe target surface. The shroud further comprises a silencing materialhaving a porous open structure. Both the sound pressure wave (noise) andgas flow which are discharged from the nozzle upon operation of theneedleless syringe are initially contained by the tubular shroud, andmust at some point pass through the silencing material. In this regard,the porosity of the silencing material is sufficient to allow escape ofgas from the shroud while retaining any and all particles that may bereflected from the target surface.

The selection of a porous material having an appropriate porosity servesto actively contain within the shroud any and all particles or particlefragments which may have been reflected from the target, while at thesame time allowing the discharged gas to pass out of the shroud throughthe material and thereby provide for a very effective silencing. Thissilencing (attenuation) feature may be enhanced by providing baffleswithin the shroud, for example by providing baffles within an annularchamber between at least a downstream section of the nozzle and asurrounding tubular portion of the shroud, or by providing steps on theouter surface of the nozzle and/or the inner surface of the shroud.

In particular aspects of the invention, the silencing material can beprovided in the form of an inner or outer sleeve which is encompassedby, or surrounds a solid shroud housing having comparatively few largevents. Alternatively, an outer wall of the shroud can be formed partlyor substantially completely of a substantially rigid porous silencingmaterial.

In other aspects of the invention, an outer substantially rigid tubularwall of the shroud is provided with one or more gas exit openings, andthe porous silencing material is a fibrous material, or a wadding offibrous material which is used to close off a plurality of gas ventsprovided at an upstream end of the shroud.

It is also a primary object of the invention to provide a needlelesssyringe having a structural silencing mechanism. Here again, the syringecomprises an elongate nozzle having an upstream end and a downstreamend. A means for releasing a high pressure flow of gas into the upstreamend of the nozzle is provided, wherein particles entrained within thereleased gas flow pass through the nozzle and exit from the downstreamend thereof for delivery to the target surface. The silencing mechanismis provided by a tubular shroud which surrounds the downstream end ofthe nozzle. The shroud has a downstream portion which extends beyond thedownstream end of the nozzle and engages the target surface to establisha positive spacing between the nozzle and the target surface. The shroudis coaxially aligned with the nozzle and is spaced apart therefrom so asto establish an intermediate chamber between the outside of the nozzleand the inside of the shroud. The shroud further comprises a silencingstructure which is in the form of a plurality of baffles which dependfrom the outside surface of the nozzle and/or from the inside surface ofthe shroud. The baffles can be arranged in a series to provide atortuous path for escaping gas to pass through on its way from thenozzle to subsequent release from the shroud. Such baffles aresufficient to attenuate the gas flow and the attendant sound pressurewave. One or more gas vents are provided at an upstream portion of theshroud for venting the gas to atmosphere. A silencing material isprovided somewhere within the path that the escaping gas must travelafter discharge from the nozzle but prior to release through the gasvent(s).

In a particular aspect of the invention, an intervening tubular housingis provided which is also coaxially aligned with the nozzle. Theintervening housing is disposed between the outside of the nozzle andthe inside of the shroud in a spaced apart relation such that an innerintermediate chamber is established between the outside of the nozzleand the inside of the intervening housing, and an outer intermediatechamber is established between the outside of the intervening housingand the inside of the shroud. The intermediate chambers each comprise aplurality of depending baffles which attenuate the flow of gasdischarged from the nozzle which then travels through the chambers forsubsequent release from the shroud. If desired, the inner and outerintermediate chambers can communicate with each other only at theirupstream portions such that gases discharged from the nozzle must travelthrough the inner chamber and then into the outer chamber for subsequentrelease from the shroud. Alternatively, the intervening housing can becomprised of a porous or perforated material which allows gas leakagebetween the inner and outer intermediate chamber substantially alongtheir entire length.

Appropriate silencing materials for use in the present inventiontypically have a small pore size with a high void volume. The small poresize helps to maximize the particle filtering aspect of the invention,while the high void volume helps to minimize impedance of the gas flow(and thus minimize backpressure). Other physical characteristics whichare taken into consideration in the selection of suitable silencingmaterials include the density, flow resistivity, and absorptioncoefficient associated with a particular material.

A suitable porous material for use as a silencing material in the shroudmay be a sintered porous media, manufactured by dry powder moulding ofparticles of, for example, polyethylene (Porex®), polypropylene, amixture of such sintered particles, or even of metal particles such asstainless steel or bronze. Other suitable porous materials for use withthe invention include foams such as a polyurethane foam. If desired, thefoam can be a reticulated polyurethane foam having a suitablecompression density. In addition, fibrous materials can be employed,such as a wadding from a fibrous open pore material.

In addition, the silencing material may have more than one porosity,e.g., a coarse pore size inner layer and a fine pore size outer layer,or vice versa, or even a graded pore size from inside to outside. Thefiner material will catch any stray particles, whereas it is believedthat the variable pore size will successively and more efficiently,attenuate and break up the sound pressure wave. In any regard, theporosity needs to be carefully chosen particularly in light of the rangeof particles sizes which are to be delivered from the needlelesssyringe. Too large a porosity will enable some stray particles to passthrough the material to atmosphere. Too small a porosity or too thick amaterial will not readily allow the gas to pass through, thus causing ahigh pressure at the tip of the syringe which may result in unwantedrecoil. Experiments with a drug having a mean particle size of 30microns have successfully been carried out using a shroud made of anopen pore solid material which has a mean pore size of 15 microns, aminimum pore size of 4 microns, and a porosity which is 55% solid and45% void. The wall thickness was 3mm.

It is an advantage of the present invention that the noise associatedwith the operation of a needleless syringe device can be substantiallyattenuated without also reducing the delivery performance of the device.It is a further advantage that any and all particles or particlefragments reflected from the target surface can be contained within thedevice shroud and not vented to atmosphere.

These and other objects, aspects, embodiments and advantages of thepresent invention will readily occur to those of ordinary skill in theart in view of the disclosure herein.

BRIEF DESCRIPTION OF THE FIGURES

Examples of syringes constructed in accordance with the presentinvention are illustrated in the accompanying drawings, in which:

FIG. 1 is an axial section through a first syringe embodiment;

FIG. 2A is an axial section through the downstream end of anothersyringe embodiment;

FIG. 2B is an enlargement of a section of FIG. 2A;

FIG. 3 is a half axial section of a third syringe embodiment;

FIG. 4 is an enlargement of a portion of the device of FIG. 4; and

FIG. 5 is an axial section of a fourth syringe embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before describing the present invention in detail, it is to beunderstood that this invention is not limited to particular needlelesssyringe device configurations as such may, of course, vary. It is alsoto be understood that the terminology used herein is for the purpose ofdescribing particular embodiments of the invention only, and is notintended to be limiting.

All publications, patents and patent applications cited herein, whethersupra or infra, are hereby incorporated by reference in their entirety.

It must be noted that, as used in this specification and the appendedclaims, the singular forms "a," "an" and "the" include plural referentsunless the content clearly dictates otherwise. Thus, for example,reference to "a silencing material" includes a mixture of two or moresuch materials, reference to "a baffle" includes two or more baffles,and the like.

A. Definitions

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which the invention pertains. The following terms areintended to be defined as indicated below.

The term "transdermal" delivery captures both transdermal (or"percutaneous") and transmucosal administration, i.e., delivery bypassage of a therapeutic agent through the skin or mucosal tissue. See,e.g., Transdermal Drug Delivery: Developmental Issues and ResearchInitiatives, Hadgraft and Guy (eds.), Marcel Dekker, Inc., (1989);Controlled Drug Delivery: Fundamentals and Applications, Robinson andLee (eds.), Marcel Dekker Inc., (1987); and Transdermal Delivery ofDrugs, Vols. 1-3, Kydonieus and Berner (eds.), CRC Press, (1987).Aspects of the invention which are described herein in the context of"transdermal" delivery, unless otherwise specified, are meant to applyto both transdermal and transmucosal delivery. That is, the needlelesssyringe devices of the invention, unless explicitly stated otherwise,should be presumed to be equally useful in carrying out transdermal andtransmucosal modes of delivery.

As used herein, the terms "therapeutic agent" and/or "particles of atherapeutic agent" intend any compound or composition of matter which,when administered to an organism (human or nonhuman animal) induces adesired pharmacologic, immunogenic, and/or physiologic effect by localand/or systemic action.

A "silencing material" encompasses any material which serves to reducethe intensity (magnitude) of a sound wave. A "sound wave" is definedherein to include any variation in atmospheric pressure that can bedetected by the human ear. The normal human ear is responsive to a widerange of pressure variations (up to a factor of about 10⁵). A commonmeasure of sound intensity is referred to in units of decibels (dB). Anysound wave that has an intensity in excess of 140 dB at any given timeis considered excessive. The silencing materials used in the presentinvention may reduce (attenuate) the intensity of sound waves by anysuitable mechanism, typically by way of their sound absorbingproperties. Dissipative silencing absorbs sound by frictional loss inthe silencing material, while reactive silencing reflects sound backtowards the source. The silencing materials of the present invention canoperate by dissipative or reactive silencing, a combination ofdissipative and reactive silencing, or by any other suitable mechanism.

A "porous" material is any material that possesses or is full of pores(minute openings or interstices) which admit the absorption or passageof a fluid or sound wave. The "porosity" of a material is the ratio ofthe volume of interstices of a material to the volume of its mass.

B. General Methods

The needleless syringe device illustrated in FIG. 1 is similar in manyrespects to that illustrated in FIGS. 1 to 3 of U.S. Pat. No. 5,630,796.Thus the device has an upper barrel portion 10 providing a reservoir 11of compressed gas, such as helium at between 30 and 80 bar. The upperbarrel portion 10 is screwed or otherwise attached to a lower barrelportion 12 providing a rupture chamber 13. The lower barrel portion 12is attached to the upper end of a nozzle 14 having aconvergent/divergent passageway. Sealed between the lower barrel portion12 and the upper end of the nozzle 14 is a cassette 15 comprising upperand lower rupturable membranes which retain a dose of a powderedtherapeutic agent therebetween.

A lower or downstream portion 16 of the nozzle is surrounded by a shroudcomprising a cylindrical portion 17 and a flared portion 18. The nozzleportion 16 has external annular ribs 19 that depend from the outersurface of the nozzle and interdigitate with internal annular ribs 20depending from the inner surface of, or otherwise disposed within theshroud portion 17.

In use, the wider end 18 of the shroud is brought into contact with thetarget surface 21, typically a skin surface, and a knob 22 is depressed.This causes a valve stem 23 to move through the reservoir 11 so that aseal 24 moves out of an exit passageway of the reservoir and releasesgas from the reservoir into the rupture chamber 13. As the pressurebuilds up, it eventually ruptures the membranes of the cassette 15,thereby releasing a high pressure gas flow through the nozzle with theparticles of therapeutic agent entrained within the flow. Theseparticles penetrate the target surface 21 to provide for the necessarytransdermal delivery. The shroud portion 18 acts as a spacer whichpositively spaces the downstream end of the nozzle from the targetsurface and allows a degree of spread in the particles as they pass fromthe nozzle to the target. The shockwave (sound wave) resulting from thegas flow rebounds from the target surface, together with a smallproportion of the particles, as suggested by the arrows 25, whereuponthe pressure and noise of the shockwave are dissipated (attenuated) andany and all particles reflected from the target surface are caughtwithin the shroud.

Dissipation of the sound wave caused by operation of the needlelesssyringe, and containment of the reflected particles can be achievedefficiently by constructing the shroud of an open pore material as shownin FIGS. 2A and 2B. FIG. 2. The shockwave is not only dissipated byhaving to traverse the tortuous path between the baffles formed by theinterdigitating ribs 19 and 20, but also by passage through the porousouter wall of the shroud. Since at least the majority of the shroud ismade up from such porous material, there is a minimum resistance tooutflow of the gas through the shroud wall, thereby minimising anyrecoil of the syringe away from the patient's skin. However, the poresize is sufficiently small to retain any particles, or particlefragments, which may rebound from the target surface.

The shroud may be moulded in complementary halves, divided by an axialplane, so as to be fitted around the nozzle and between the ribs 19 uponassembly with the nozzle, prior to the halves being bonded together. Theupper end of the shroud portion 17 can be bonded, snap fitted, orotherwise secured to the upper portion 14 of the nozzle by any suitableattachment means.

FIG. 2A shows a modification in which the baffles provided by the ribs19 and 20 are omitted, but the shroud is provided with a step 26 betweenits smaller and larger diameter portions 17A and 18A surrounding thenozzle 16A. This step acts to some degree in providing adequate reboundsurfaces to initiate dissipation of the shockwave and deceleration ofany rebounding particles.

The needleless syringe device depicted in FIGS. 3 and 4 has a particledelivery mechanism which is similar to that of the device of FIG. 1.Components of the syringe device of FIGS. 3 and 4, which, although of adifferent detailed shape, have the same function as reference numerals10-15 and 22-24 in FIG. 1, are given the reference numerals 10B-15B and22B-24B. A repetitive description of these parts is therefore deemedunnecessary.

An essential difference between the needleless syringe of FIG. 3 and thesyringe depicted in FIG. 1 resides in the construction and operation ofthe tubular shroud. More particularly, the tubular shroud in the deviceof FIG. 3 extends up almost to the top of the reservoir 11B andcomprises a number of components which are welded, screwed, or otherwiseattached together. These components include an upper portion 27, whichis attached to the top of the upper barrel portion 10B, an outercylindrical wall 28, an intervening cylindrical housing 29, and anoutlet portion 30, into which the downstream end of the nozzle 14Bopens. The outlet portion 30 is shown as having a generallyfrustoconical portion merging into a substantially cylindrical portion.This outlet section acts as a spacer which, when pressed against thetarget surface, positively spaces the downstream end of the nozzle adistance of about 10 mm away from the target surface.

The intervening tubular housing 29 divides the annular space between theinner surface of the shroud wall 28 and the outer surface of the nozzle14B into two coaxial inner and outer intermediate chambers 31 and 32.These intermediate chambers are typically arranged in series, beinginterconnected at their top (upstream) ends by a hollow interior 33 ofthe upper portion 27 of the shroud. In addition, each intermediatechamber provides a tortuous path for a gas flowing therethrough byvirtue of a plurality of discontinuous baffles 34. These baffles candepend from an outer surface of the nozzle 14B, a surface of theintervening housing 29, and/or from an inner surface of the outercylindrical wall 28 of the shroud. In each intermediate chamber (31 and32), these baffles can be axially spaced about 10 mm apart and consistsof an annular disc with two diametrically opposed slots, each occupyingabout 60° of arc, with the slots in one baffle 90° out of alignment withthose in the adjacent baffle in the same chamber. As can be seen inFIGS. 3 and 4, the lower end of the inner intermediate chamber 31 isopen to the interior of the outlet portion 30. The lower end of theouter intermediate chamber 32 communicates with a ring of gas vent exitopenings 35, disposed within a nut member 36, wherein each opening isprovided by a bore having a diameter of about 2 mm. Located between theshroud wall 28 and the nut member 36 is an annular ring of cotton-woolwadding 37 which isolates the interior of the chamber 32 from theopenings 35.

In use, the flow of gas discharged from the nozzle, and any particlesreflected from the target surface, are caused to pass up through theinner intermediate chamber 31, following a tortuous path around thebaffles 34, travel through into the upstream end of the outerintermediate chamber 32 (via the hollow interior portion 33 of the uppershroud portion 27), whereafter the gas and possibly some of thereflected particles pass down again along a tortuous path around thebaffles 34. The gas is then able to permeate through the wadding 37 andhence be vented to atmosphere through the openings 35 while the wadding37 captures any residual particles.

The larger the volume of the intermediate chambers 31 and 32 the better.However, it has been found in practice that a vented volume of about 80ml works very well.

The function of the silencer mechanism of FIGS. 3 and 4 can be enhancedif there is provision for leakage of gas from the inner intermediatechamber 31 to the outer intermediate chamber 32 through the interveningtubular housing 29. This may be provided for by forming a portion of thehousing 29 from a porous silencing material, or by includingperforations 38 in the housing. One particular embodiment of theinvention uses an arrangement of four equalangularly spaced holesthrough the housing 29 between each pair of baffles 34. Those holeswhich are physically further away from the target surface (upstream,i.e., higher up in FIGS. 3 and 4), are preferably a little larger thanthose lower down. For example, holes in the upper half may be 2 mm indiameter and those in the lower half 1.5 mm in diameter. As analternative to the holes 38, the intervening tubular housing 29 can becomprised substantially of an open pore silencing material.

Experiments show that the construction shown in FIGS. 3 and 4 enablesthe noise level to be reduced to an acceptable level of below 80 dB at 1m distance with an applied load of 20 N.

The syringe of FIG. 5 has a particle delivery mechanism similar to thoseof the devices described herein above, and repetitive description ofthese features is thus deemed unnecessary. Here again, substantiallyidentical components are numbered alike. In the instant syringe, yetanother construction and operation of the shroud is used to silence theoperation of the device.

Referring particularly to FIG. 5, the shroud comprises a number ofcomponents which are welded, screwed, or otherwise attached together.These components include a substantially rigid outer wall 50 having aplurality of gas exit openings 52 disposed therein. A silencing material54 is disposed between an inner surface of the outer wall 50 and anouter surface of the nozzle 14C. An outlet portion 30B, into which thedownstream end of the nozzle 14C opens acts as a spacer which, whenpressed against the target surface, positively spaces the downstream endof the nozzle a distance of about 10 mm away from the target surface.The silencing material 54 is comprised of a suitable open pore material,preferably from a sheet of reticulated polyurethane foam.

In use, the flow of gas discharged from the nozzle, and any particlesreflected from the target surface, must pass up through the silencingmaterial 54, whereupon the gas is able to permeate through the silencingmaterial 54 and hence be vented to atmosphere through the openings 52,while the silencing material 54 prevents escape of any reflectedparticles.

Accordingly, novel silenced needleless syringe device are disclosed.Although preferred embodiments of the subject invention have beendescribed in some detail, it is understood that obvious variations canbe made without departing from the spirit and the scope of the inventionas defined by the appended claims.

We claim:
 1. A needleless syringe for delivering particles to a targetsurface, said syringe comprising:(a) an elongate nozzle having anupstream end and a downstream end; (b) means for releasing a highpressure flow of gas into the upstream end of the nozzle, whereinparticles entrained within the released gas flow pass through the nozzleand exit from the downstream end thereof for delivery to the targetsurface; and (c) a tubular shroud surrounding the downstream end of thenozzle, wherein said shroud has a downstream portion which extendsbeyond the downstream end of the nozzle and engages the target surface,and further wherein said shroud comprises a silencing material having aporous open structure the porosity of which is sufficient to allowescape of gas from the shroud while retaining any particles reflectedfrom the target surface.
 2. The syringe of claim 1, wherein the shroudis coaxially aligned with the nozzle and spaced apart from the nozzle toprovide an intermediate chamber between the outside of the nozzle andthe inside of the shroud, said intermediate chamber comprising aplurality of baffles depending from the outside of the nozzle or theinside of the shroud, wherein said baffles are sufficient to attenuatethe flow of gas discharged from the nozzle which passes through theintermediate chamber for subsequent release from the shroud.
 3. Thesyringe of claim 2 further comprising an intervening tubular housingwhich is coaxially aligned with the nozzle and disposed between theoutside of the nozzle and the inside of the shroud, said interveninghousing being spaced apart from the nozzle and the shroud to establishinner and outer intermediate chambers, wherein said intermediatechambers have a plurality of depending baffles which are sufficient toattenuate the flow of gas discharged from the nozzle which passesthrough the inner and outer intermediate chambers for subsequent releasefrom the shroud.
 4. The syringe of claim 3, wherein the interveningtubular housing is porous or perforated to allow gas leakage between theinner and outer intermediate chambers.
 5. The syringe of claim 4,wherein the intervening tubular housing is formed from an open porefoam.
 6. The syringe of claim 3, wherein the inner and outerintermediate chambers are in fluid communication with each other only atupstream portions thereof such that gas discharged from the nozzle mustpass through the inner chamber and then into the outer chamber forsubsequent release from the shroud.
 7. The syringe of claim 1, whereinthe silencing material has a small pore size and a high void volume. 8.The syringe of claim 7, wherein the silencing material is a foam.
 9. Thesyringe of claim 8, wherein the silencing material is a polyurethanefoam.
 10. The syringe of claim 1, wherein the shroud comprises asubstantially rigid outer wall having one or more gas exit openings, andthe silencing material is comprised of an open pore material disposedbetween the outer wall and the nozzle.
 11. The syringe of claim 10,wherein the silencing material is comprised of an open pore foam whichsubstantially surrounds the downstream end of the nozzle and issandwiched between the nozzle and the outer wall of the shroud.
 12. Thesyringe of claim 11, wherein the silencing material is a polyurethanefoam.
 13. The syringe of claim 1, wherein the shroud comprises asubstantially rigid outer wall having one or more gas exit openings, andthe silencing material is comprised of a fibrous open pore materialdisposed within or closing said one or more gas exit openings.
 14. Thesyringe of claim 13; wherein the silencing material is a wadding offibrous open pore material.
 15. The syringe of claim 1, wherein thesilencing material is a substantially rigid open pore material.
 16. Thesyringe of claim 1, wherein the silencing material has a fibrousstructure.
 17. The syringe of claim 1, wherein the shroud comprises aninner or outer wall which is substantially comprised of the silencingmaterial.